Conventional axial piston pumps (i.e., "Thoma" pump) are often used in high pressure applications. For example, in a hydraulically actuated electronically controlled united injector (HEUI) fuel control system, a high pressure, axial piston oil pump typically supplies the diesel injectors with 3,000-4,000 psi engine oil for hydraulic operation. This high pressure oil pump is charged with low pressurized fluid from another pump, typically the engine's oil pump. Conventionally, an auxiliary pump, the engine's fuel pump, is driven by the HEUI pump. The fuel pump transfers fluid from the fuel tank to the injectors for consumption by the engine and typically pumps at approximately 20-50 psi.
In many of these applications, the high pressure pump also drives a low pressure pump. A typical arrangement is illustrated in prior art FIG. 1 in which an input shaft 10 is splined to a rotatable cylinder 12 having circumferentially spaced bores containing pistons 13. One end of each piston is ball shaped and received in a socket receptacle formed as a slipper 14 which, in turn, contacts an end face of a stationary swash plate 15. Rotation of Input shaft 10 rotates cylinder 12 to cause pistons 13 to axially reciprocate in their bores by slipper contact with swash plate 15 while fluid intake and exhaust of pressurized fluid is through conventional kidney shaped intake/outtake ports 16. Press fitted onto the tail end of input shaft 10 is a cam 18 which acts as an eccentric to drive a prime mover 19 of an auxiliary pump 20.
In vehicular applications, space is at a premium and is often a determining factor in the OEM's selection process, especially for mature technologies such as that embodied in an axial piston pump. In the arrangement illustrated in FIG. 1, the addition of auxiliary pump 20 onto the tail end of input shaft 10 increases the length of the pump assembly. A more subtle point is that an eccentric lift is provided at a tail extension of the input shaft which requires that the input shaft be soundly journaled so as not only to unduly transmit loads to the high pressure pump but also to insure against any axial run out of the shaft which could potentially adversely affect the smoothness of the lift motion of prime mover 19, especially if cam 18 wears. In the prior art pump of FIG. 1, front and rear ball bearings 21 journal input shaft 10 and internal and external retainer rings 22, 23 (lubricated) prevent shaft run out. The FIG. 1 arrangement has proven to be durable and commercially acceptable. Its length, however, is increased by auxiliary pump 20 and its cost must reflect the bearing arrangement.
In SAE Technical Paper 2000-01-0687, entitled "Development of a Variable-Displacement, Rail-Pressure Supply Pump for a Dimethyl Ether" by James C. McCandless, Ho Teng and Jeffrey B. Schneyer presented Mar. 6-9 2000, an axial piston pump is disclosed in which, like the parent application, a rotating swash plate/stationary cylinder is disclosed. In the pump disclosed in the SAE paper, the circumferential edge of the swash plate is used to control valving to the axial piston pump. Like FIG. 1, the input shaft of the SAE disclosed pump is journaled in ball bearings. Additionally, springs in the piston bores are used to maintain slippers in contact with the swash plate.